In the field of electrical devices such as electric power tools and light fittings, phase control or reverse phase control of power to a load such as an AC (Alternating Current) motor or lighting load is widely performed. For example, JP 2009-12149A and JP 08-154392A disclose control apparatuses for an electric power tool or an AC motor that perform phase control of an AC motor, using a triac or an SSR (Solid State Relay) as a switching element.
In the case where phase control or reverse phase control of an AC load is performed in an electrical device, electromagnetic noise arises due to the sudden change in current at the time of switching. With an electrical device such an electric power tool in which current flow to the AC load is high, the adverse effects on surrounding electrical devices and the human body because of the considerable amounts of electromagnetic noise caused by switching is of particular concern.
JP 11-161346A discloses a phase control apparatus for performing phase control or reverse phase control using two MOSFETs (Metal-Oxide Semiconductor Field-Effect Transistors) connected in series in opposite directions. In recent years, transistors capable of controlling high current such as MOSFETs and IGBTs (Insulated Gate Bipolar Transistors) have become popular in the power electronics field. Compared with triacs and SSRs, transistors are advantageous in reducing the change in current at the time of switching. Hence, even with phase control or reverse phase control of electrical devices (e.g., electric power tools) in which a comparatively high current flows to the load, suppression of electromagnetic noise at the time of switching is conceivable by using a transistor capable of controlling high current as a switching element.
In the case where phase control or reverse phase control using a transistor capable of controlling high current is performed in an electrical device that operates at high current, a comparatively high constant voltage used as a gate or base drive voltage of the transistor needs to be generated and applied to the gate or base of the transistor. A phase control apparatus shown in FIG. 2 of JP 11-161346A uses a gate power supply that uses a transformer to obtain a gate drive voltage from an AC voltage. However, such a gate power supply unit is not preferable in terms of requiring a comparatively large installation area and being costly and heavy.
Also, with a phase control apparatus shown in FIG. 8 of JP 11-161346A, a series circuit of the AC power supply and the load is connected between input terminals of a diode bridge, although full-wave rectifying an AC voltage applied between these terminals with a diode bridge does not allow a stable high DC (Direct Current) voltage to be obtained. Hence, the configuration of this phase control apparatus is not preferable for phase control or reverse phase control using a transistor capable of controlling high current.
If the gate or base drive voltage of a transistor is generated from an AC voltage using half-wave rectification rather than full-wave rectification, it should be possible to generate the gate or base drive voltage using a comparatively simple circuit configuration. However, in order to perform phase control or reverse phase control stably and accurately, the gate or base drive voltage needs to be stable. In view of this, the gate or base drive voltage preferably is generated by full-wave rectifying an AC voltage.